Just restarting spacewalk will not rebuild the cache. The cache
regenration should have started when you're centos client checked in.
check your /var/cache/rhn/repodata and the cache should be generated
in a bit. The cache generation for repodata is triggered only when
there is a change to the channel state(through means like rhnpush,
satsync, api or ui) or when a client checks in and cache is missing.
Check back in a bit and cache should be there,
~ Prad